Refractory metal barrier in semiconductor devices

ABSTRACT

Gate metallization structures and methods for semiconductor devices are disclosed, wherein a refractory metal barrier is implemented to provide performance improvements. Transistor devices are disclosed having a compound semiconductor substrate and an electron-beam evaporated gate structure including a layer of tantalum nitride (TaNx), a layer of titanium (Ti) and a layer of gold (Au).

RELATED APPLICATION

This application claims priority to U.S. Provisional Application Nos.61/897,814, filed on Oct. 30, 2013, entitled REFRACTORY METAL BARRIER INSEMICONDUCTOR DEVICES, and 61/897,802, filed Oct. 30, 2013, entitledSYSTEMS, DEVICES AND METHODS RELATED TO REACTIVE EVAPORATION OFREFRACTORY MATERIALS, the disclosures of which are hereby incorporatedby reference in their entirety.

BACKGROUND

1. Field

The present disclosure generally relates to metal gate structures forsemiconductor devices.

2. Description of Related Art

Gate structures in certain semiconductor devices may include variousmetals in addition to, or as an alternative to, polysilicon. Althoughmetal gate structures can enable higher clock speeds, lower powerconsumption, as well as providing possibly other benefits over certainalternative configurations, gate sinking and other issues can lead toperformance degradation in certain embodiments.

SUMMARY

In some implementations, the present disclosure relates to a transistordevice that includes a compound semiconductor substrate and anelectron-beam evaporated gate structure including a layer of tantalumnitride (TaNx), a layer of titanium (Ti) and a layer of gold (Au). Thevalue of the variable x associated with the TaN compound may be a valuebetween 0.0 and 0.5, such as a value between 0.2 and 0.3. In certainembodiments, the TaNx compound is Ta2N.

In certain embodiments, a thickness of the TaNx layer is betweenapproximately 100 and 300 angstroms. For example, the thickness of theTaNx layer may be approximately 150 angstroms. Furthermore, the gatestructure may be a component of a field-effect transistor (FET). Forexample, the gate structure may be a component of a pseudomorphic highelectron mobility transistor (pHEMT).

In certain embodiments, the layer of TaNx has a thickness betweenapproximately 100 and 400 angstroms. The layer of TaNx may be configuredto provide a diffusion barrier between the layer of Au and the compoundsemiconductor substrate. The gate structure may be configured tosubstantially prevent gate sinking into the compound semiconductorsubstrate when subjected to a processing temperature at or aboveapproximately 300° C.

In certain embodiments, the gate structure includes an additional layerof TaNx and an additional layer of Ti. In certain embodiments, none ofthe layer of TaNx, the layer of Ti, or the layer of Au is formed usingsputter deposition. Furthermore, the gate structure may or may notinclude platinum or palladium.

Certain embodiments disclosed herein provide a Schottky diode includinga compound semiconductor substrate and a metal structure disposed on thecompound semiconductor substrate including a first metal layer and arefractory metal layer disposed between the first metal layer and thecompound semiconductor substrate, the refractory metal layer configuredto inhibit diffusion of the first metal layer into the compoundsemiconductor substrate.

The refractory metal layer may include tantalum nitride (TaNx) and thefirst metal layer may include gold (Au). The Schottky diode may furtherinclude a second metal layer disposed between the first metal layer andthe refractory metal layer. In addition, the Schottky diode may includea passivation layer disposed above the first metal layer. For example,the passivation layer may be titanium. In certain embodiments, thesecond metal layer is titanium.

In certain embodiments, the refractory metal layer is betweenapproximately 100 and 300 angstroms in thickness. For example, thethickness of the refractory metal layer may be approximately 150angstroms. The refractory metal layer may be formed using an evaporationprocess.

Certain embodiments disclosed herein provide a method of fabricating asemiconductor device, the method including providing a compoundsemiconductor substrate, evaporating a layer of tantalum nitride (TaNx)onto the compound semiconductor in a first processing step and using afirst tool, evaporating a layer of titanium (Ti) onto the layer of TaNxin the first processing step and using the first tool, and evaporating alayer of gold (Au) onto the layer of Ti in the first processing step andusing the first tool.

BRIEF DESCRIPTION OF THE DRAWINGS

Various embodiments are depicted in the accompanying drawings forillustrative purposes, and should in no way be interpreted as limitingthe scope of the inventions. In addition, various features of differentdisclosed embodiments can be combined to form additional embodiments,which are part of this disclosure. Throughout the drawings, referencenumbers may be reused to indicate correspondence between referenceelements.

FIG. 1 schematically shows that in some implementations, a device havingone or more features as described herein can be formed on a compoundsemiconductor substrate such as a wafer.

FIG. 2 shows an example of the device of FIG. 1, including a gatestructure in accordance with one or more embodiments.

FIG. 3 shows another example of the device of FIG. 1, including a gatestructure in accordance with one or more embodiments.

FIG. 4 shows an example of a metal stack structure.

FIG. 5 schematically shows an example configuration of a metal stackstructure.

FIG. 6 schematically shows an example configuration of a metal stackstructure including a refractory metal layer according to one or moreembodiments.

FIGS. 7A-7C show examples of transistor structures where one or morefeatures of the present disclosure can be implemented.

FIG. 8 shows an example of a Schottky diode structure where one or morefeatures of the present disclosure can be implemented.

FIGS. 9-10 show a process that can be implemented to fabricate the gatestructure of FIG. 6.

FIG. 11 shows a process that can be implemented to fabricate a diehaving one or more metalized structures as described herein.

FIGS. 12A and 12B schematically depict a module having one or more dies.

FIG. 13 schematically depicts a radio-frequency (RF) device thatincludes one or more dies and/or one or more modules having one or morefeatures as described herein.

FIG. 14 is a block diagram showing an embodiment of a wireless device inaccordance with one or more features of the present disclosure.

FIGS. 15A-15D are graphs illustrating potential performance achievablethrough implementation of refractory barrier gate stacks according toone or more embodiments disclosed herein.

DETAILED DESCRIPTION

The headings provided herein are for convenience only and do notnecessarily affect the scope or meaning of the claimed invention.

Although certain preferred embodiments and examples are disclosed below,inventive subject matter extends beyond the specifically disclosedembodiments to other alternative embodiments and/or uses and tomodifications and equivalents thereof. Thus, the scope of the claimsthat may arise herefrom is not limited by any of the particularembodiments described below. For example, in any method or processdisclosed herein, the acts or operations of the method or process may beperformed in any suitable sequence and are not necessarily limited toany particular disclosed sequence. Various operations may be describedas multiple discrete operations in turn, in a manner that may be helpfulin understanding certain embodiments; however, the order of descriptionshould not be construed to imply that these operations are orderdependent. Additionally, the structures, systems, and/or devicesdescribed herein may be embodied as integrated components or as separatecomponents. For purposes of comparing various embodiments, certainaspects and advantages of these embodiments are described. Notnecessarily all such aspects or advantages are achieved by anyparticular embodiment. Thus, for example, various embodiments may becarried out in a manner that achieves or optimizes one advantage orgroup of advantages as taught herein without necessarily achieving otheraspects or advantages as may also be taught or suggested herein.

Overview

Various metallic elements and/or refractory alloys may be utilized inintegrated circuits (ICs) for gate metallization in semiconductordevices. In particular, metal gate stacks may be useful in connectionwith gallium arsenide (GaAs) and/or other III-V compound semiconductors.For example, Schottky contacts may be used as gate metal for GaAs andindium phosphide (InP)-based substrates. Gates used for forming certainmetal-semiconductor field effect transistors (MESFETs),high-electron-mobility transistors (HEMTs), and other semiconductordevices may comprise metallic structures that form Schottky barriercontacts.

Materials such as gold (Au), aluminum (Al) and silver (Ag) can providesubstantially good Schottky contacts to GaAs. However, heat treatment atapproximately 300° C., for example, which may be required for subsequentprocessing, can result in inter-reaction, wherein gate metal diffusesinto the semiconductor and/or gallium diffuses out of the semiconductor.Metal spiking, change of barrier height, and/or deviation from idealrectifying behavior may result from such inter-reaction. Metals such astitanium (Ti) or tungsten (W) may advantageously provide relatively moreinert interface properties. These metals can be used as part of amultilayer Schottky diode or transistor gate contact to provide at leasta partial barrier between the primary gate metal and the semiconductor.

The present disclosure describes semiconductor gate metallizationschemes that may provide reduced gate sinking and/or othermetal-semiconductor inter-reaction when utilized in semiconductordevices. FIG. 1 schematically shows that in some implementations, adevice 112 having one or more features as described herein can be formedon a compound semiconductor substrate such as a wafer 110. In certainembodiments, a device 112 formed on a compound semiconductor substrate(such as a wafer 110) can include one or more multilayer gate metalstructures having one or more features as described herein. Variousexamples disclosed herein are described in the context of galliumarsenide (GaAs) compound semiconductor on which such gate structures canbe implemented. However, it will be understood that such gate featurescan also be implemented on other compound semiconductors. For example,compound semiconductors such as InP, GaN, InGaP, and InGaAs can beutilized as a substrate on which gate structures having one or morefeatures as described herein are formed.

It will also be understood that although various examples herein aredescribed in the context of compound semiconductors, one or morefeatures of the present disclosure can also be implemented on elementalsemiconductors. For example, elemental semiconductors such as siliconand/or germanium can be utilized as a substrate on which gate structureshaving one or more features as described herein are formed.

FIG. 2 shows an example of a device as shown in of FIG. 1, including agate structure in accordance with one or more embodiments. FIG. 2illustrates a transistor device 212 including a source region, a drainregion, and a gate structure. The gate structure may allow for thepassage of electrical signals from a semiconductor substrate to anexternal contact (not shown) through the gate structure. The gatestructure may be a multilayer gate structure including one or morelayers as described below with respect to FIGS. 4-6.

FIG. 3 shows another example of a device as shown in FIG. 1, including agate structure in accordance with one or more embodiments. In certainembodiments, the device 312 is a pseudomorphic HEMT (pHEMT). The device312 includes a gate structure that is fabricated using a lift-offapproach. For example, the device 312 may be fabricated at leastpartially according to the following process: photoresist is applied tothe wafer 110, and is patterned using a mask and photolithography. Theexposed film is then developed and a metal film is evaporated onto thepatterned wafer, before the remaining photoresist is removed to leavemetallic contacts. In certain embodiments, gate structure is disposed onetched WN_(x). Details of the gate structures of FIGS. 2 and 3 aredescribed below.

Multilayer Metalization

Ti, W, and/or other elements can be utilized in gate metallization toprovide at least a partial barrier between the primary gate metal of agate structure and the semiconductor. However, in certain embodiments,Ti can form TiAs at temperatures above approximately 550° C. when incontact with GaAs. Furthermore, Ti may not prevent gold diffusion intoGaAs over extended periods of time even at relatively lowertemperatures. Certain digital circuit embodiments include WSi_(x) orWN_(x)-type sputtered gates, where the sheet resistance of the gatelayer can be high. However, for submicron gate layers used in analog ormixed signal circuits, it may be undesirable or impractical for the linesheet resistance to be too high. Certain embodiments include platinum(Pt)/Au Schottky gates. However, such schemes may present instabilitiesbased on the intermixing of Pt with GaAs at approximately 300° C. orgreater and at contact alloying temperatures, which may be required formaking ohmic contacts (e.g., around 380° C. in rapid thermalprocessing). In view of the various potential shortcomings of these gateconfigurations, relatively more complex multilayer gate metallization(e.g., patterned by lift-off) may be necessary or desirable for certainapplications.

FIG. 4 illustrates an example multilayer gate structure 400 inaccordance with one or more embodiments. The gate structure 400 may beformed over a compound semiconductor substrate 402 and may include aSchottky metal contact element 404 configured to provide a rectifyingcontact between the gate metal 410 and the underlying semiconductor 402.Such rectifying contacts may be useful in certain semiconductor devices,such as Schottky diodes, or certain transistor devices. Whether a givenmetal-semiconductor junction provides an ohmic contact or Schottkybarrier can depend on the Schottky barrier height, φB, of the junction.For a sufficiently large Schottky barrier height, where φB issignificantly higher than the thermal energy kT, the semiconductor candeplete near the metal and behave as a Schottky barrier. For lowerSchottky barrier heights, the semiconductor is not depleted and insteadforms an ohmic contact to the metal. Certain embodiments disclosedherein provide for a metal-semiconductor junction having a sufficientlylarge barrier height to provide a rectifying Schottky contact.

The gate structure 400 may include an additional barrier element 406, aprimary gate metal 410, as well as a relatively inert passivationelement 412 disposed above the primary metal 410 forprotecting/preserving the gate structure. In certain embodiments, thecontact layer 404, barrier layer 406, and primary metal layer 410comprise titanium, platinum, and gold, respectively. Titanium may bedesirable as a base layer that contacts the semiconductor because itgenerally forms a high-quality Schottky junction due to its workfunction and barrier height characteristics. Furthermore, titanium canadhere relatively well to the semiconductor, though the electricalconductivity is relatively poor (e.g., bulk resistivity of approximately4×10⁻⁵ Ω/cm). Gold may be added to at least partially compensate for therelatively poor electrical conductivity of the titanium. However, asdescribed above, gold can migrate and diffuse through titanium into thedevice under certain conditions, which can degrade device performance.

Multiple-layer metallization, such as Ti/Pt/Au or Ti/W/Au, may beadopted to at least partially obviate the effects of interaction betweenthe gate metal and the semiconductor. In certain embodiments, sputteredTi/W (e.g., approximately 10-20% Ti) may be used to separate a goldlayer from the Schottky contact layer 404. In certain embodiments, acontact via placed on top of the Schottky contact may be required ordesirable. Other types of barrier layers may be preferable or desirablein certain embodiments.

The barrier layer 406 may be sandwiched between the primary metal 410and the contact layer 404 to at least partially prevent diffusion intothe semiconductor 402. In certain embodiments, platinum may be desirableas a barrier metal, although other types of elements, such as palladiumor other transition metal, may be used to alleviate cost or other issuesassociated with platinum. As further shown in FIG. 4, the gate structure400 can also include a passivation layer 412 formed over the metal layer410 comprising, for example, titanium.

FIG. 5 schematically shows an example configuration of a metal stackstructure implementing a multilayer scheme. The embodiment of FIG. 5provides a triple-layer, evaporated Ti/Pt/Au metallization scheme, whichmay be advantageous for Schottky and gate metallization. In certainembodiments, Pt may be replaced by Palladium (Pd), which, althoughpossibly not providing as effective a barrier as Pt, may often be morecheaply obtainable.

Although Ti/Pt/Au metallization, as shown in FIG. 5, can provide variousbenefits when utilized in connection with III-V semiconductor devices,such a metallization scheme may be susceptible to some degree to gatemetal sinking, as described above, which can place limits on thepost-gate process thermal budget. Gate sinking, which involves thediffusion of gate metal into the top semiconductor under the gate metaland the channel, can result in changing of the threshold voltage and/orzero-bias drain current (I_(DSS)) during post gate formation processingand can ultimately prove fatal to a semiconductor device. As more layersof metal interconnect are added, the stability of the gate, andconsequently the threshold voltage and transistor current, may beadversely affected. In addition to threshold control, reliability alsomay become questionable, and larger area devices may fail in practicedue to metal sinking. Devices may degrade during packaging (involvingmore thermal processing during which diffusion can continue), as well asduring the life of the device while in field.

In pHEMT circuits, the problems discussed above may cause loss ofperformance and/or gate shorting due to diffusion of gold into thechannel. In more complex technologies like the BiFET and BiHEMT, ternarysemiconductor compounds (like InGaAs) may be used for channels, andSchottky layers on top (for example, InGaP) may be added. However, gatediffusion and sinking may remain, and possibly worsen, depending uponthe composition of the ternary semiconductor.

Refractory Metal Barriers

In certain embodiments, gate sinking may be at least partiallyalleviated through the use of a refractory metal for the Schottkycontact. Refractory metals generally exhibit relatively high meltingpoints (e.g., above approximately 2000° C.). Furthermore, refractorymetals provide relatively inert chemical properties, as well asrelatively high density. Such characteristics of refractor metals mayprovide reduced diffusion and/or gate sinking when disposed as part of agate structure. In certain embodiments, tantalum (Ta) and/or tungsten(W), and/or compounds thereof, may be used in a gate stack to suppressdiffusion and gate sinking. In other embodiments, niobium (Nb),molybdenum (Mo), or rhenium (Re) may be used.

FIG. 6 schematically shows an example configuration of a metal stackstructure 600 including a refractory metal layer according to one ormore embodiments. The gate structure 600 includes an Au gate with abilayer Schottky metal and refractory barrier comprising Ti and TaN_(x).For example a TaN_(x) compound may be Ta2N, or other compoundcorresponding to a value of x between 0.0-0.5. In certain embodiments,the gate 600 comprises WSi_(x), WN_(x) or WSi_(x)N_(y). Such gatematerials may be used for IC processing for digital circuits where largecurrent-carrying capability is not needed. The use of TaN_(x) incombination with Ti may substantially limit interaction between the Aulayer and the underlying semiconductor.

The thickness d3 of the gold layer may be between approximately1000-10,000 Å, or up to 2 μm or thicker. In certain embodiments, thedimension d2, which represents the thickness of the intermediate Tilayer, may be approximately 100-800 Å. Further, the dimension d4, whichrepresents the thickness of the optional top Ti layer, may beapproximately 30-200 Å in thickness. With respect to the refractorybarrier layer, the thickness d1 may be approximately 100-300 Å. Incertain embodiments, if the thickness d1 of TaNx is too low, it will notprovide a sufficient barrier. However, if the thickness is too high, theSchottky barrier properties of TaNx may take over, and the FET gatecharacteristics may shift. In certain embodiments, the thickness d1 isapproximately 150 Å.

In certain embodiments, T-gate formation methods are utilized to cladrefractory elements/compounds with gold for use in situations wherelarger current, lower resistance gates are needed. Although sputterdeposition is a commonly used metal deposition technique, for radiofrequency (RF) circuits, the use of sputtered WN_(x)-type materials maybe limited.

In certain embodiments, Ni/Au or Pd/Au bilayers may also be used.However, such bilayers may suffer limitations caused by gate sinkingwhen applied to large scale production.

The gate structure of FIG. 6, and/or gate structures embodyingprinciples associated therewith and described herein, may provide one ormore of the following benefits and/or characteristics: a barriermaterial that can be deposited with a high process latitude, wherein gasflow rates, background oxygen levels, and/or the like may not need becontrolled to high precision; the gate structure fabrication process maynot require multiple steps and/or multiple tools; a material stack thatcan be deposited using ordinary e-beam production process; the layerstructure may allow for the avoidance of sputter deposited layers, whichmay need to be etched off in processes incompatible with III-V compoundsemiconductor processing; a material stack that can used for gate sizesas small as traditional photolithography is capable of, and as large asneeded or desired without a substantial risk of inter-diffusion, such asin large area devices where the probability of inter-diffusion may beincreased; a gate layer structure that is not prone to processvariations due to variable surface conditions; a metal layer structurethat avoids use of expensive precious metals like platinum (Pt) orpalladium (Pd); and a relatively simple, one photolithography processfor multilayer gates, consisting of a highly conducting gold-like layerwith a refractory layer underneath touching the semiconductor channel.

Refractory Metallization in FET/pHEMT Devices

FIGS. 7A and 7B illustrate an example of a multilayer gate structurecomprising refractory metallization embodied in a FET device. Forexample, the device of FIGS. 7A and 7B may be a MOSFET, or the like.While FIG. 7A illustrates a cross-sectional view, FIG. 7B provides a topview, as shown.

An example of a multilayer gate structure comprising refractorymetallization can be further described in the context of an examplepHEMT structure 150 shown in FIG. 7C. The pHEMT structure of FIG. 7C canbe formed on a semi-insulating GaAs substrate. A GaAs buffer layer maybe formed over the substrate. A GaAs—AlGaAs superlattice region may beformed over the buffer layer. The superlattice region can include aplurality of GaAs and AlGaAs layers. A first AlGaAs barrier layer may beformed over the superlattice region. A portion of the AlGaAs barrierlayer is shown to be doped with silicon (dashed line). An InGaAs channellayer may be formed over the first AlGaAs barrier layer. In certainembodiments, a second AlGaAs barrier layer is formed over the InGaAschannel layer, which may also be doped with silicon.

Additional layers to provide the source, drain, and gate functionalitiescan be formed. For example, an n+ GaAs layer is shown to be formed belowthe gate structure. In certain embodiments, a TaN/Au gate may be used ina T-gate for the pHEMT, as shown. Gates of HEMT devices are oftenSchottky contacts. Therefore, rectifying gate structures comprisingbuilt-in refractory barriers as described herein may advantageously beutilized in pHEMT devices, or other HEMT or transistor devices. The gateof the pHEMT may comprise an Au gate with a bilayer Schottky metal andrefractory barrier comprising Ti and TaN_(x). Furthermore, the gate mayhave any suitable refractory barrier/gate structure as described herein.

Refractory Metallization in Schottky Diodes

FIG. 8 shows an example of a Schottky diode structure 800 where one ormore features of the present disclosure can be implemented. The diode800 may be formed on an N+ doped semiconductor substrate 834, which isin ohmic contact with a metal layer 860. The metal layer 860 may provideelectrical contact for a cathode of the diode 800. In certainembodiments, a lightly doped epitaxial n-type blocking layer 832 isformed above the substrate. The doping and thickness of the epitaxiallayer 832 may be selected to achieve a desired blocking voltage.

The diode 800 includes a metal structure 882 through which electricalcommunication with the anode is achieved. In certain embodiments, themetal structure 882 may comprise a multilayer configuration, asdescribed in association with one or more embodiments herein. Thestructure 882 may include a refractory metal Schottky barriercomprising, for example, TaN_(x) and/or Ti, as well as a Au gate layer.The structure 882 may further include a passivation layer on top of thegold layer, such as a layer of Ti or the like.

The diode 800 may include one or more isolation trenches or regions 818,such as in the form of a ring at least partially surrounding theSchottky junction. Such isolation region 818 may be implanted prior todeposition of the Schottky metal. The isolation region 818 may helpfield crowding at the periphery of the metal in the blocking state,which may reduce the blocking voltage.

Schottky diodes utilizing refractory metallization may be incorporatedin various RF components. For example, Schottky diodes constructed asdisclosed herein may be used as components for frequency mixers, RFpower detector circuits, or other devices or components.

Processing

In certain embodiments, TaN barriers, or other refractory metalbarriers, may be deposited using a sputter deposition process. However,sputter deposition can cause damage to the semiconductor under the metalunder certain conditions, which may need to be annealed out. In certainembodiments, low-resistivity gates are desirable, wherein etching may benecessary to achieve the desired resistivity characteristics due to thethickness of the sputtered metal. However, etch processes may not becompatible with certain III-V semiconductor processes, such as certainheterojunction bipolar transistor (HBT), enhanced bipolar field effecttransistor (BiFET) and integrated HBT/pHEMT (BiHEMT) processes becauseof the presence of silicon nitride on the wafers, which can becomeetched off, thereby exposing sensitive devices/components to ion damageand requiring additional processing to deposit new passivation. Inaddition, lift-off processes may also be incompatible with thicksputtered metal films. If a layer thick enough to provide effectivebarrier characteristics is used as a lower gate layer (e.g., contactingthe semiconductor channel or the top Schottky layer), then it may benecessary for a highly-conductive layer, such as gold or the like, to beadded on top. Therefore, multilayer gate and processing methods designedto achieve such structure, partially patterned by sputter-lift-off andpartially by lift-off, may be desirable, though such methods may besubstantially complicated and require more steps.

Surfaces of III-V binary or ternary compounds can be substantiallycomplex, which can make consistent surface reproduction difficult inproduction conditions. Furthermore, the surfaces can change uponexposure to air, such as between the cleaning step and the actualdeposition of the metal. Therefore, the nature of the semiconductorsurface in large volume production may vary substantially. Even whenprecautions are taken, the state of surface oxidation may change and theintermixing of the gate metal and the semiconductor can vary as aresult. Under these conditions, adhesion metals, such as Ti, Pt, and Pd,may diffuse to varying degrees. Certain embodiments disclosed hereinadvantageously provide a refractory built-in barrier, such as TaN_(x),(x being a value between 0.0-0.5, such as between 0.2-0.3), evaporatedby electron beam evaporation, deposited in the same evaporation step asone or more other layers of a gate structure.

Although certain embodiments are described herein in the context of GaAschannel layers, other types of semiconductor substrates may be useddepending on preference or other factors. For example, inepitaxially-grown structures, particularly with respect tohetero-structures, ternary compounds like AlGaAs may be used as a toplayer. Forming a top layer under the Schottky metal that is less proneto process issues and intermixing may be desirable. For example, withrespect to pHEMT and other epi-structure devices, AlGaAs, InGaP and/orother compounds, such as In, AlP etc., may be used.

Certain embodiments disclosed herein provide for the use of oxides infabricating III-V MOSFETs. For such purpose, it may be desirable toutilize a semiconductor material that forms a good clean oxide, similarto silicon, and also allows for formation of a metal gate thereon thatis stable. Such applications may utilize, for example, Ti/Pt/Au gatemetallization, as described herein. Alternatively, Ni/Au or Pd/Austructures may be utilized. Instability caused by sinking that may beexperienced using such structures may be used to adjust thresholdvoltages by submitting the wafers to thermal cycles to control thethreshold voltage. In certain embodiments, Molybdenum (Mo) may be usedto provide increased stability. Furthermore, Mo/Au gates may providerelatively high-speed. However, evaporating Mo in production can bedifficult, as the melting point is approximately 2890° C. In addition,molybdenum is used as a common crucible liner in e-beam evaporationsystems.

TaNx can provide certain advantages over other refractory materials,such as Mo and W. For example, Mo generally does not form dry etchablecompound like TaNx. Furthermore, the film stress cannot be reduced bythe addition of other element or compound formation, so films can besubject to peeling. Tungsten can form a nitride that is etchable.However, evaporation of tungsten may not be practical, even withelectron beam methods. Various systems and method for evaporating metalsin accordance with one or more embodiments disclosed herein aredescribed in greater detail in U.S. application Ser. No. 14/527,577,entitled “SYSTEMS, DEVICES AND METHODS RELATED TO REACTIVE EVAPORATIONOF REFRACTORY MATERIALS” and filed on even date herewith, as well asU.S. Pat. No. 8,022,448 entitled “APPARATUS AND METHODS FOR EVAPORATIONINCLUDING TEST WAFER HOLDER” and filed on Oct. 5, 2010, both of whichare incorporated herein by reference in their entirety.

Example Method for Fabrication of Example Metal Stack

Certain embodiments disclosed herein provide for evaporation of TaNx byelectron beam evaporation, which may present challenges in view of thevery high melting point of tantalum. With an evaporated TaNx film, astandard lift-off process may be used to enable better control ofstructure line width.

FIG. 9 shows an embodiment of a process 900 that can be implemented tofabricate the example metal stack of FIGS. 6-8. FIG. 10 shows variousstages of fabrication of the metal stack. At block 932, a compoundsemiconductor substrate (1002) such as GaAs may be provided. The process900 further includes forming a refractory barrier over the substrate atblock 934. The refractory barrier may comprise TaN, or other compound,and may be deposited using an electron beam evaporation process, asdescribed herein.

At block 936, a first Ti layer (1064) can be formed over the refractorymetal barrier (1062). In some implementations, the first Ti layer (1064)can be formed by an electron-beam evaporation deposition process.

At block 940, a Au layer (1010) can be formed over the first Ti layer(1064). In some implementations, the Au layer (1010) can be formed by anelectron-beam evaporation deposition process. A tungsten (W) ormolybdenum (Mo) crucible can be used for the evaporation of Au. Use ofsuch a liner can provide advantageous features such as reducing thermalcontact to the hearth (which is water cooled in some situations),thereby allowing a higher deposition rate for a given beam power. Otherdeposition methods and configurations can also be utilized.

At block 948, a second Ti layer (1070) can be formed over the Au layer(1010). In some implementations, the second Ti layer (176) can be formedby an electron-beam evaporation deposition process similar to theexample associated with the first Ti layer (1064). Other depositionmethods and configurations can also be utilized.

Although evaporating tantalum can present certain challenges, electronbeam evaporated TaNx film can provide certain advantages over sputteredTaN film. Thin layers of Ta_(x)N_(1-x), denoted herein in certaincontexts as TaNx, may be used in electronic components built from III-Vsemiconductors, among other devices. Such layers may be deposited bysputter deposition, for ease of control of composition and TCR forcertain devices where uniformity and reproducibility are significantconsiderations. Dry etching of TaN films may be relatively difficult andmay involve relatively harsh chemistry. In addition, it may berelatively difficult to achieve high selectivity with chlorine chemistrywhen the film is deposited over, for example, silicon nitride. In III-Vsemiconductor circuits, such as circuits formed on GaAs substrates,lift-off patterning process may be preferred. Patterning over siliconnitride may be performed by dielectric assisted lift-off (DAL) process,where the nitride is etched off and the TaNx film sits on GaAs. Such aprocess may be associated with certain drawbacks. Alternatively,evaporation methodology may allow for the use of standard lift-offtechnique to define the relevant device or component, therebyeliminating the need for DAL. In addition, the TaN can be placed onSiNx, which may improve active device leakage current. Furthermore, withthe ability to form the TaNx film directly on silicon nitride, the TCRof the TaN may be considerably more stable and the device may be lesssusceptible to leakage through the GaAs substrate. In certainembodiments, a large bias is applied to the mask, which may place alimit on device-to-device spacing. Certain embodiments disclosed hereinprovide an alternative method for depositing TaNx film by means ofelectron beam evaporation with nitrogen incorporation in the processchamber. By optimizing the input parameters, a stable TaNx film can beachieved matching the desired properties of the sputtered TaN film.

E-beam evaporators may be used for TaNx deposition with minimaladditional hardware. N2 may be plumbed into the process chamber using afeed-through, tubing and the gas distribution controlled via a MFC and apressure gauge. Tantalum, which is a refractory metal with a very highmelting point, can be relatively challenging to evaporate. High powermay be required to evaporate tantalum metal. However, with proper meltsetup and maintaining a low deposition rate, the process may beestablished with reasonable power control. The process is manufacturablefor film thickness providing a sheet resistance of approximately 50ohms/sq., which may require only a few hundred angstroms.

N2 may be incorporated into the tantalum deposition throughout theentire process layer. In certain embodiments, the properties of the TaNxfilm may be controlled by varying the nitrogen content of the film.Tantalum may be readily reactive to the nitrogen resulting in a fairlywide range of N2 flow with stable and repeatable film resistance andstress. The optimum deposition conditions may ultimately be determinedbased on finding a N2 gas flow in a range that is not sensitive to smallchanges and achieving low film stress. Within this range, thecomposition of the film can be chosen to achieve a resulting film thatmeets the properties for the film.

In some embodiments, metalized structures having one or more features asdescribed herein can be implemented in a compound semiconductor wafer,which in turn can yield a plurality of dies. Each of such dies caninclude an integrated circuit configured to perform a number offunctions. FIG. 11 shows a process 1100 that can be implemented tofabricate a die (1210 in FIG. 12A) having one or more metalizedstructures as described herein. At block 1102, one or more semiconductordevices can be formed on a compound semiconductor substrate such asGaAs. At block 1104, one or more metalized gate structures can be formedfor such devices. At block 1106, a die having an integrated circuit thatincludes the metalized gate structures can be formed.

In some embodiments, a die having one or more metalized gate structuresas described herein can be implemented in a module. FIG. 12A shows anexample of such a module 1220, and FIG. 12B shows a block diagramrepresentation of the same. The module 1220 can include a die 1210,which may be mounted on a packaging substrate 1222, and can be protectedby an overmold structure. Electrical connections to and from the die1210 can be facilitated by connections 1226 such as wirebonds. Suchwirebonds can be interconnected to connection pads 1228 formed on themodule so as to facilitate connectivity to other modules and/or externalcomponents.

In some embodiments, the module 1220 can also include one or moresurface-mount devices (SMD) mounted on the packaging substrate 1222 andconfigured to facilitate and/or complement the functionality of theintegrated circuits in the die 1210. In some embodiments, the module1220 can also include one or more packaging structures to, for example,provide protection and facilitate easier handling of the module 1220.Such a packaging structure can include an overmold formed over thepackaging substrate 1222 and dimensioned to substantially encapsulatethe various circuits and components thereon. It will be understood thatalthough the module 1220 is described in the context of wirebond-basedelectrical connections, one or more features of the present disclosurecan also be implemented in other packaging configurations, includingflip-chip configurations.

In some embodiments, a module or a die having one or more metal gatestructures as described herein can be implemented in a radio-frequency(RF) device. FIG. 13 shows an example of such an RF device 1330. The RFdevice 1330 can include a module 1320 similar to the module described inreference to FIGS. 12A and 12B. In some implementations, such a modulecan facilitate operation of a transmit/receive circuit 1332 and anantenna 1334. In some embodiments, the module 1320 can be configured toprovide, for example, amplification of RF signals, switching of RFsignals, and/or other RF related functions.

In some implementations, a device and/or a circuit having one or morefeatures described herein can be included in a wireless device. Such adevice and/or a circuit can be implemented directly in the wirelessdevice, in a modular form as described herein, or in some combinationthereof. In some embodiments, such a wireless device can include, forexample, a cellular phone, a smart-phone, a hand-held wireless devicewith or without phone functionality, a wireless tablet, etc.

FIG. 14 schematically depicts an example wireless device 1400 having oneor more advantageous features described herein. One or more PAs 110 areshown, which can facilitate, for example, multi-band operation of thewireless device 1400. In embodiments where the PAs and their matchingcircuits are packaged into a module, such a module can be represented bya dashed box 301.

The PAs 110 can receive their respective RF signals from a transceiver1410 that can be configured and operated in known manners to generate RFsignals to be amplified and transmitted, and to process receivedsignals. The transceiver 1410 is shown to interact with a basebandsub-system 1408 that is configured to provide conversion between dataand/or voice signals suitable for a user and RF signals suitable for thetransceiver 1410. The transceiver 1410 is also shown to be connected toa power management component 1406 that is configured to manage power forthe operation of the wireless device. Such power management can alsocontrol operations of the baseband sub-system 1408 and the module 301.

The baseband sub-system 1408 is shown to be connected to a userinterface 1402 to facilitate various input and output of voice and/ordata provided to and received from the user. The baseband sub-system1408 can also be connected to a memory 1404 that is configured to storedata and/or instructions to facilitate the operation of the wirelessdevice, and/or to provide storage of information for the user.

In the example wireless device 1400, outputs of the PAs 110 are shown tobe matched (via match circuits 420) and routed to an antenna 1416 viatheir respective duplexers 1412 a-1412 d and a band-selection switch1414. The band-selection switch 1414 can include, for example, asingle-pole-multiple-throw (e.g., SP4T) switch to allow selection of anoperating band (e.g., Band 2). In some embodiments, each duplexer 1412can allow transmit and receive operations to be performed simultaneouslyusing a common antenna (e.g., 1416). In FIG. 14, received signals areshown to be routed to “Rx” paths (not shown) that can include, forexample, a low-noise amplifier (LNA).

A number of other wireless device configurations can utilize one or morefeatures described herein. For example, a wireless device does not needto be a multi-band device. In another example, a wireless device caninclude additional antennas such as diversity antenna, and additionalconnectivity features such as Wi-Fi, Bluetooth, and GPS. The componentsdescribed above in connection with FIG. 14 and wireless device 1400 areprovided as examples, and are non-limiting. Moreover, the variousillustrated components may be combined into fewer components, orseparated into additional components. For example, baseband sub-system1408 can be at least partially combined with the transceiver 1410. Asanother example, the transceiver 1410 can be split into separatereceiver and transmitter portions.

The wireless device 1400 may include one or more pseudomorphic highelectron mobility transistors (pHEMT), metal semiconductor field effecttransistors (MESFET), heterojunction bipolar transistors (HBT), bipolarFETs (BiFET), bipolar HEMTs (BiHEMT), and/or surface acoustic wave (SAW)devices (e.g., SAW filter or resonator) that can include or be connectedto gate structures having one or more features as described herein. Itwill be understood that other semiconductor devices utilized in wirelessdevices can also benefit from such gate structures. It will also beunderstood that semiconductor devices associated with other types ofelectronic devices can also benefit from gate structures having one ormore features as described herein.

FIGS. 15A-D provide graphs illustrating potential performanceimprovements with respect to Idss that may be achievable throughimplementation of refractory barrier gate stacks in connection withcertain embodiments disclosed herein. In particular, FIGS. 15A and Bshow Idss and device passage rates over time for devices not utilizingTaN barrier layers. FIGS. 15C and D show possible improved performancefor TaN devices. The graphs of FIGS. 15A-D illustrate the effect oftemperature on FET matching versus gate size according to certainembodiments.

Other Embodiments

Depending on the embodiment, certain acts, events, or functions of anyof the processes or algorithms described herein can be performed in adifferent sequence, may be added, merged, or left out altogether. Thus,in certain embodiments, not all described acts or events are necessaryfor the practice of the processes. Moreover, in certain embodiments,acts or events may be performed concurrently, e.g., throughmulti-threaded processing, interrupt processing, or via multipleprocessors or processor cores, rather than sequentially.

Conditional language used herein, such as, among others, “can,” “could,”“might,” “may,” “e.g.,” and the like, unless specifically statedotherwise, or otherwise understood within the context as used, isintended in its ordinary sense and is generally intended to convey thatcertain embodiments include, while other embodiments do not include,certain features, elements and/or steps. Thus, such conditional languageis not generally intended to imply that features, elements and/or stepsare in any way required for one or more embodiments or that one or moreembodiments necessarily include logic for deciding, with or withoutauthor input or prompting, whether these features, elements and/or stepsare included or are to be performed in any particular embodiment. Theterms “comprising,” “including,” “having,” and the like are synonymous,are used in their ordinary sense, and are used inclusively, in anopen-ended fashion, and do not exclude additional elements, features,acts, operations, and so forth. Also, the term “or” is used in itsinclusive sense (and not in its exclusive sense) so that when used, forexample, to connect a list of elements, the term “or” means one, some,or all of the elements in the list. Conjunctive language such as thephrase “at least one of X, Y and Z,” unless specifically statedotherwise, is understood with the context as used in general to conveythat an item, term, element, etc. may be either X, Y or Z. Thus, suchconjunctive language is not generally intended to imply that certainembodiments require at least one of X, at least one of Y and at leastone of Z to each be present.

It should be appreciated that in the above description of embodiments,various features are sometimes grouped together in a single embodiment,figure, or description thereof for the purpose of streamlining thedisclosure and aiding in the understanding of one or more of the variousinventive aspects. This method of disclosure, however, is not to beinterpreted as reflecting an intention that any claim require morefeatures than are expressly recited in that claim. Moreover, anycomponents, features, or steps illustrated and/or described in aparticular embodiment herein can be applied to or used with any otherembodiment(s). Further, no component, feature, step, or group ofcomponents, features, or steps are necessary or indispensable for eachembodiment. Thus, it is intended that the scope of the inventions hereindisclosed and claimed below should not be limited by the particularembodiments described above, but should be determined only by a fairreading of the claims that follow.

What is claimed is:
 1. A transistor device comprising: a compoundsemiconductor substrate; and a gate structure including a siliconnitride (SiN_(x)) layer deposited on the compound semiconductorsubstrate, an electron beam evaporated tantalum nitride (TaN_(x)) layerhaving a thickness less than 300 angstroms and deposited on the SiN_(x)layer, a first electron beam evaporated titanium (Ti) layer deposited onthe TaN_(x) layer, an electron beam evaporated gold (Au) layer depositedon the first Ti layer, and a second electron beam evaporated Ti layerdeposited on the Au layer.
 2. The transistor device of claim 1 wherein xis a value between 0.0 and 0.5.
 3. The transistor device of claim 2wherein x is a value between 0.2 and 0.3.
 4. The transistor device ofclaim 1 wherein TaN_(x) is Ta2N.
 5. The transistor device of claim 1wherein a thickness of the TaNx layer is between 100 and 300 angstroms.6. The transistor device of claim 5 wherein the thickness of the TaNxlayer is approximately 150 angstroms.
 7. The transistor device of claim5 wherein the TaN_(x) layer is configured to provide a diffusion barrierbetween the Au layer and the compound semiconductor substrate.
 8. Thetransistor device of claim 1 wherein the gate structure is a componentof a field-effect transistor (FET).
 9. The transistor device of claim 1wherein the gate structure is a component of a pseudomorphic highelectron mobility transistor (pHEMT).
 10. The transistor device of claim1 wherein the gate structure is configured to substantially prevent gatesinking into the compound semiconductor substrate when subjected to aprocessing temperature at or above 300° C.
 11. The transistor device ofclaim 1 wherein the gate structure includes an additional layer ofTaN_(x).
 12. The transistor device of claim 1 wherein none of theTaN_(x) layer, the first Ti layer, the second Ti layer or the Au layeris formed using sputter deposition.
 13. A Schottky diode comprising: acompound semiconductor substrate; and a metal structure disposed on thecompound semiconductor substrate, the metal structure including anelectron beam evaporated refractory metal layer having a thickness lessthan 300 angstroms disposed on the semiconductor substrate, a firstelectron beam evaporated titanium (Ti) layer disposed on the refractorymetal layer, an electron beam evaporated gold (Au) layer formed on thefirst Ti layer, and a second electron beam evaporated Ti layer formed onthe Au layer, the refractory metal layer configured to inhibit diffusionof the Au layer into the compound semiconductor substrate.
 14. TheSchottky diode of claim 13 wherein the refractory metal layer includestantalum nitride (TaN_(x)).
 15. The Schottky diode of claim 13 whereinthe second Ti layer provides a passivation layer for the metalstructure.
 16. The Schottky diode of claim 13 wherein a thickness of therefractory metal layer is between 100 and 300 angstroms.
 17. TheSchottky diode of claim 16 wherein the thickness of the refractory metallayer is approximately 150 angstroms.
 18. The Schottky diode of claim 13wherein the refractory metal layer is formed using an evaporationprocess.
 19. A method of fabricating a semiconductor device, the methodcomprising: providing a compound semiconductor substrate; evaporating alayer of tantalum nitride (TaN_(x)) onto the compound semiconductorsubstrate using electron beam evaporation in a first processing step andusing a first tool; evaporating a first layer of titanium (Ti) onto thelayer of TaN_(x) using electron beam evaporation in the first processingstep and using the first tool; evaporating a layer of gold (Au) onto thefirst layer of Ti using electron beam evaporation in the firstprocessing step and using the first tool; and evaporating a second layerof Ti onto the layer of Au using electron beam evaporation.